The invention concerns a tuning system for a satellite receiver, especially one capable of receiving and processing television signals transmitted in digital form.
Satellite television receiving systems usually comprise an xe2x80x9coutdoor unitxe2x80x9d including a dish-like receiving antenna and a xe2x80x9cblockxe2x80x9d converter, and an xe2x80x9cindoor unitxe2x80x9d including a tuner and a signal processing section. The block converter converts the entire range (xe2x80x9cblockxe2x80x9d) of relatively high frequency RF signals transmitted by a satellite to a more manageable, lower range of frequencies.
In a conventional satellite television transmission system television information is transmitted in analog form and the RF signals transmitted by the satellite are in the C (e.g., 3.7 to 4.2 GHz) and Ku (e.g., 11.7 to 14.2 GHz) bands. The RF signal received from the satellite by the antenna of the receiving system are converted by the block converter to the L band (e.g., 900 to 2000 MHz). An RF filter section of the tuner of the indoor unit selects the one of the RF signals received from the block converter corresponding to the selected channel, and a mixer/local oscillator section of the tuner converts the selected RF signal to a lower, intermediate frequency (IF) range for filtering and demodulation.
In newer satellite television systems, such as the DirecTv(trademark) operated by the Hughes Corporation of California, television information is transmitted in digital form. The RF signals are transmitted by the satellite in the Ku band, and are converted by the block converter to the L band. The frequency range of the RF signals transmitted by the satellite is somewhat smaller (e.g., between 12.2 and 12.7 GHz) than that for the analog satellite television system, and the frequency range of RF signals produced by the block converter is accordingly somewhat smaller (e.g., between 950 and 1450 MHz).
As in the analog satellite television receiving systems, the RF signal corresponding to the selected channel has to be reduced in frequency to an IF frequency range for filtering and demodulation. In a digital satellite receiver, in addition to the normal IF filtering for selecting the desired RF signal and rejecting unwanted RF signals, it is desirable that the IF filter perform what is known as xe2x80x9csymbol shapingxe2x80x9d to reduce decoding errors due to xe2x80x9cinter-symbol interferencexe2x80x9d caused by bandwidth limitations.
The conversion stage of the block converter of the outdoor unit usually includes a local oscillator which is not stabilized against variations of temperature and age. The result is that the frequency of the local oscillator signal of the block converter changes, causing a corresponding change or offset of the frequencies of the carrier signals of the RF signals received by the tuner of the indoor unit. As a consequence, the frequency of the IF signal produced by the tuner also changes or is offset from its nominal value. If the frequency of the IF signal changes too far from its nominal value, the digital signals modulated on the IF signal cannot be properly demodulated and the information they represent cannot be properly reconstructed. To overcome this problem, the offset frequency is monitored and an offset added to nominal frequency command to change the local oscillator of the tuner to center the signal in the IF filter.
Part of the invention resides in the recognition that the RF signals received from the LNB and the corresponding IF signal produced by the tuner may be offset in frequency due to reasons other than a frequency drift of the oscillator of the LNB. More specifically, satellite transponder frequency adjustments may be made by the satellite transmission system operator to reduce the possibility of interference between carrier signals. By way of example, a transponder frequency may be changed by as much as +/xe2x88x922 MHz. The transponder frequency adjustments cause the RF signals received from the LNB and the corresponding IF signal produced by the tuner to have a frequency offset.
The present invention concerns provisions for tuning frequency offsets due to the adjustment of individual transponder frequencies by the satellite transmission system operator. These provisions allow the transmission frequencies of the transponders to be adjusted by the satellite transmission system operator without unduly increasing the time for the indoor unit to acquire the digital signal when a new channel is selected. Briefly, the tuning system measures and stores individual transponder originated frequency offsets Any offset due to LNB frequency drift is added to all of the transponder frequency offsets as a xe2x80x9cglobalxe2x80x9d offset. An individual transponder offset is updated if it is not possible to tune a transponder frequency or if the successful acquisition required a frequency offset greater than a predetermined threshold or is a broad frequency search was required to acquire the signal. These and other aspects of the transponder frequency offset provisions are described below.
These and other aspects of the invention will be described in detail with reference to the accompanying Drawing.